1. Field of the Invention
The present invention relates to a comparator circuit, for example, a comparator circuit for an FSK (frequency shift keying) demodulation circuit, and a comparator circuit suited to binarizing an ASK (amplitude shift keying) signal demodulated using an RSSI (received signal strength indicator).
2. Background Art
In demodulating an FSK signal, it is common practice to remove a signal component included in the FSK signal to extract a DC component, deal with the extracted component as an average voltage of the FSK signal, and compare this voltage as a threshold voltage with the FSK signal, to generate binarized data.
In general, the average voltage is obtained using a primary low-pass filter including a resistor and a capacitor. The cut-off frequency of the low-pass filter is required to be sufficiently lower than the frequency of the signal component included in the FSK signal. For this reason, the capacitance of the capacitor becomes large, time for charging and discharging becomes long, the rise of the average voltage becomes slow, and hence it takes a long time to obtain the binarized data. Therefore, when the input signal voltage Vin varies, the average voltage Vref cannot adapt to it rapidly, due to the time constant of the primary low-pass filter including the resistor and the capacitor.
Particularly in a system which operates on battery, shortening the rise time is important for extending battery life, and therefore a charge/discharge circuit that rapidly charges and discharges the capacitor is required. To realize this, there has been used a technique that involves charging and discharging the capacitor by use of a diode.
However, with this arrangement, the signal voltage applied during charging or discharging the capacitor is constrained to the forward voltage VF of the diode. That is, in a case where Vin−Vref, which is obtained by subtracting the average voltage Vref from the input signal voltage Vin, is a positive value, the capacitor is charged when this value becomes larger than the forward voltage VF, whereas in a case where Vin−Vref is a negative value, the capacitor is discharged when this value becomes smaller than the forward voltage −VF. However, it is impossible to set arbitrarily a voltage at which charging/discharging is started, and therefore this voltage is fixed at ±VF.
Further, the forward voltage VF of a diode is about as large as 0.6 V, and it has been difficult to adapt to the reduction of the signal amplitude resulting from the recent trend toward low power-supply voltages. In addition, the forward voltage VF depends on temperature (−2 mV/° C.), and it has been difficult to obtain high reliability.
JP-A H5-252009 (KOKAI) describes an example of a slice level generating circuit including first and second operational amplifiers and first and second diodes.
Further, in demodulating an FSK signal or an AKS signal, it is common practice to generate an average voltage of the FSK signal or the AKS signal as a threshold, and compare the FSK signal or the AKS signal with the threshold, to binarize the FSK signal or the AKS signal.
In generating the average voltage, there is available a method that involves using a primary low-pass filter including a resistor and a capacitor. In this method that involves using the primary low-pass filter, if the cut-off frequency is set at a value sufficiently lower than the signal frequency to prevent the attenuation of the FSK signal or the ASK signal, the rise of the average voltage becomes slow and it takes a long time before the binarization is completed.
For this reason, when the method is applied to a system that operates on battery, the consumption of the battery becomes severe, the battery needs to be replaced frequently, and hence the usability of the system becomes poor.